1. Field of the Invention
The present invention relates to a vestigial sideband/quadrature amplitude modulation (VSB/QAM) shared receiver, and more particularly, to an apparatus and method for eliminating VSB pilot tone during VSB modulation in a VSB/QAM shared receiving system.
2. Description of the Related Art
Typical VSB/QAM shared receivers denote receivers designed so as to receive a VSB signal as well as a QAM signal. VSB/QAM shared receivers do not need to include a separate VSB receiver for VSB signals, so that hardware volume of receivers is reduced. Here, reception of VSB signals in a VSB/QAM shared receiver is accomplished using a technique of restoring the center frequency in a frequency band of a VSB signal to a direct current (DC), instead of using an existing method of restoring a pilot tone into a DC. Thus, a demodulated VSB signal is detected in a similar method to a method of detecting an offset QAM signal.
FIG. 1 is a block diagram of the typical VSB/QAM shared receiver described above. In the operation of the typical VSB/QAM shared receiver, referring to FIG. 1, a received signal is a signal which is received from an antenna, a cable, an optical fiber or the like, for example, and primarily demodulated to a center frequency. The received signal is applied to a first mixer 102 for mixing the received signal with a co-phase center carrier frequency, and a second mixer 110 for mixing the received signal with a center carrier frequency which has been 90° phase-shifted by a Hilbert filter 109. The center carrier frequency is supplied from a carrier oscillator 120 which is controlled in response to a carrier frequency control signal which is output by a decoder 118. The output of the first mixer 102 and the output of the second mixer 110 are applied to first and second analog-to-digital converters (ADC) 104 and 112, respectively, which operate in response to a control clock signal which is output by a clock oscillator 122.
Digital data streams from the first and second ADCs 104 and 112 are applied to first and second data slicers 108 and 116, respectively, via first and second BPFs 106 and 114, respectively, or via digital high-pass filters. The first and second data slicers 108 and 116 supply a binary pulse if a data value from received eye patterns exceeds a critical value for distinguishing between noise or other possible data values. The decoder 118 supplies a carrier restoration control signal to the carrier oscillator 120 to control a carrier frequency (fcs) from the carrier oscillator 120. Also, the decoder 118 supplies a clock restoration control signal to the clock oscillator 122 to control the clock oscillator 122 which operates the first and second ADCs 104 and 112 according to a data rate. The clock oscillator 122 can be a voltage controlled oscillator (VCO) which is controlled in response to a voltage clock frequency control signal-produced to maintain an appropriate frequency without misalignment of a received data clock, a symbol clock and all of other related data clocks.
As shown in FIG. 1, the conventional VSB/QAM shared receiver demodulates both a VSB signal and a QAM signal to reduce its hardware volume. However, in the conventional VSB/QAM shared receiver, the frequency of an oscillator which is oscillated within a demodulation unit is different from what is shown in FIG. 2 showing a typical VSB signal spectrum. That is, in the conventional VSB/QAM shared receiver, the frequency of an oscillator is oscillated to synchronize with the center frequency (fcs) of a VSB signal other than a pilot tone frequency (fp), and an offset-QAM signal is also demodulated together with the VSB signal.
A conventional VSB dedicated receiver demodulates a received VSB signal so that the frequency of the received VSB signal synchronizes with the same frequency as a pilot tone frequency. At this time, a pilot tone, which is shown as a DC component by demodulation, can be easily eliminated similar to removal of DC. However, when a VSB signal is demodulated in synchronization with the center frequency as described above, the conventional VSB/QAM shared receiver does not restore a pilot tone signal to a DC value. The pilot tone signal is converted into a discrete frequency tone having a frequency corresponding to a difference between the center frequency and a pilot tone frequency, and there remains a particular frequency tone within a demodulated VSB signal.